Research On The Steam Oxidation Resistance Of 2.25Cr-1Mo Heat-resistant Steel For Nuclear Energy

Nuclear energy,as a kind of green,environmental protection and sustainable clean energy,is being used in more and more countries and regions.2.25Cr-1Mo steel was selected as the main structural material for steam generators in the generation IV nuclear power system,in order to cope with the complex and harsh working environment in nuclear power reactors,the smelting process and performance of the steel grade have been improved,at present,industrial trial production has been carried out in China.Resistance to steam oxidation of the steel is one of the important indicators of material reliability.Studies the resistance to steam oxidation of 2.25Cr-1Mo steel for nuclear power,and provides a technical basis for the application of this material.Oxidation resistance of 2.25Cr-1Mo steels with Cr content(mass fraction,%)of2.37 and 1.99 were studied at 460?,480?,500?in 0.1 MPa steam.Phases of oxide products were identified by XRD and EDS,morphology and structure of scales were characterized by SEM and EDS,its oxidation kinetics was studied by weight-added method and the long-term oxide film thickness of the material was calculated.The effect of temperature and Cr content on the steam oxidation performance were studied,the growth mechanism of the oxidation scale of 2.25Cr-1Mo steel and the reasons for the formation of holes and peeling of oxide scale.Results show:The oxidation kinetic curve of 2.25Cr-1Mo steel conforms to the cubic law and has good long-term resistance to steam oxidation.The crystal grains on the surface of the oxide scale grow from cube-shape,and then a layer of fine grains grows again on the surface of the Cubic grains,and some fine grains develop into whiskers.The oxide scale has a double-layered structure,in which the inner layer is denser and the main phase is(Fe,Cr)₃O₄ spinel,while the outer layer is loose and porous and the main phase are Fe₃O₄ and Fe₂O₃.At 500?,the oxide scale of Cr2.37%experimental steel is the thickest,but meets the design requirements.The thickness of the oxide scale of Cr1.99%experimental steel is greater than the design requirements,and the oxide scale and the substrate are peeled.With the increase of temperature,the diffusion rate of ions in the oxide scale is increased,and the oxidation rate,weight gain and the thickness of the oxide scale are increased.Cr forms(Fe,Cr)₃O₄ spinel in the inner layer of the oxide scale,which hinders the diffusion of Fe and O ions in the oxide scale,with the increase of Cr content,the hindrance increase of the inner layer of the oxide scale to the diffusion of ions,the oxidation rate decrease,the oxidation gain hand the thickness of the oxide scale decrease.The oxide scale grew up through the diffusion transfer of Fe and O ions in the oxide scale.The holes in the oxide scale are related to the vacancy defects in the surface layer and instability decomposition the accumulation and collapse of vacancy defects of Fe₃O₄.The peeling of oxide scale and the process of experiment,thermal stress in oxide scale is related to growth stress.